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What Is Digestion?

Digestion is the process of transforming food into basic nutrients that can be absorbed and used by the body. Just as a car needs gas to move, our bodies require fuel to function and perform daily activities. Once absorbed, these nutrients can be used to fuel cellular growth, fight infections, and provide energy to our muscles and brain. Without the process of digestion, the nutrients we initially consume in food would be useless.

Before food even enters the mouth, a cascade of events involving the nervous system and a variety of hormones are set off based on a variety of stimulus:

• cognition: thoughts about food can start the flow of saliva
• sound: hearing a description of the meal
• odor: smells stimulate a hunger response and influence its taste
• appearance: seeing the food
• taste: begins as the food enters your mouth and also how it feels

Once food enters the mouth, it begins to break down into smaller units in both mechanical and chemical processes. The mechanical process begins in the mouth with chewing. From there, involuntary muscle contractions are used to move food mixtures along the tract, a process referred to as peristalsis.

However, it should also be noted that chemical aids may be present along the various stages of digestion. Enzymes, acid, bile, and mucus are all chemical means the body has at its disposal to further aid in breaking down food. Notably, enzymes are proteins that catalyze (speed up) chemical reactions. Relative to digestion, enzymes catalyze the chemical reactions required to break down food particles into smaller parts, which prepares the nutrients for absorption.

Absorption

Nutrient absorption occurs across the wall of the gastrointestinal (GI) tract—a long hollow tube consisting of several layers of tissue that begins with the mouth and ends at the anus. The mucosa (intestinal wall) is the inner-most layer, and it is made of absorptive cells and glands. Circular and longitudinal muscles comprise the outer layers, both of which function to mix and move food along the GI tract. Notably, circular bands of muscle are most often found where one part of the tract connects to another, serving as valves to control the flow of the food particles.

As shown in Figure 2.1, there are three processes the body can use to move nutrients from the GI tract into the blood/lymph system and eventually into the cells:

1. Passive diffusion: Substances move easily in and out of cells without the use of energy. Nutrients move from high to low concentrations.

2. Facilitated diffusion: No energy is required, but a special protein carrier is required to help substances cross in or out of the cell. Nutrients move from high to low concentrations.

3. Active transport: Energy is required to move substances in or out of the cell. Nutrients move from low to high concentrations.

neither passive nor facilitated diffusion require an initial input of energy to transport nutrients across a membrane. However, a specialized carrier (or gatekeeper) protein is required for facilitated diffusion. In both cases, nutrients naturally move from an area of high concentration to an area of low concentration. Unlike passive transport, active transport not only requires an input of energy, it also moves nutrients against the natural gradient, moving from areas of low concentration to areas of high concentration.

As shown in Figure 2.3, the GI tract can be divided into six main parts:

1 - mouth
2 - esophagus
3 - stomach
4 - small intestine
5 - large intestine
6 - rectum

Additionally, four main organs produce and secrete substances that aid in digestion, but they are not part of the GI tract:

1 - salivary glands
2 - liver
3 - gallbladder
4 - pancreas

Mouth (the beginning of the GI tract): The function of the mouth is to alter the food particles to prepare them to be swallowed. As previously described, both mechanical and chemical digestions take place in the mouth. Mechanical digestion occurs as the teeth break the food into smaller particles. The salivary glands are then stimulated to secrete saliva. Saliva is a watery fluid containing (1) a lubricant (mucus) and (2) enzymes to prepare food for the next step in the GI tract.

Mucus mixes with food, lubricating the particles and making it easier to swallow.

Enzymes released in the mouth have specific functions:

• Salivary amylase breaks down starches.
• Lingual lipase breaks down fats.

As an important component of the mouth, the tongue gives us the ability to taste food. The taste buds on the tongue provide the pleasure sensation we experience when we eat. There are five flavors that can be identified:

1 – sweet
2 – salty
3 – sour
4 – bitter
5 – umami (a meaty flavor)

Esophagus (the tube that connects the throat with the stomach): The role of the esophagus is to transport food to the stomach. During the swallowing process, food lands on a flap of tissue called the epiglottis.

The epiglottis folds down over the trachea, or wind pipe, during swallowing to prevent food from entering the trachea. Without this mechanism, food would accidentally enter the trachea, resulting in choking. Once in the esophagus, the nervous system sends signals to the surrounding muscles in the GI tract to initiate peristalsis, which propels the food forward.

The sphincter muscle is a circular muscle located at the end of the esophagus. Once food passes through it on the way to the stomach, the muscle constricts (closes) and prevents the backflow of the stomach contents into the esophagus.

Stomach (has a four-cup capacity and can hold food for two to three hours, or longer, until it is ready to pass into the small intestine): The stomach’s role is to store, mix, dissolve, and continue the digestion of food. As a continuation of the chemical digestion that began in the mouth, food particles in the stomach are mixed with gastric juice and enzymes. The acid in the gastric juice prepares proteins for digestion and activates enzymes.

• Pepsin is an enzyme that breaks down proteins.
• Gastric lipase is an enzyme that begins fat digestion.

Chyme is the resulting substance when food particles are mixed with stomach acids and enzymes. Chyme is a very watery mixture that slowly empties from the stomach into the small intestine. Depending on the size of the meal consumed, this process occurs over a period of one to four hours.

Gastrin is the hormone responsible for controlling the concentration of acid in the stomach. Gastrin is produced when we begin to think about eating foods as well as during the actual ingestion of food. In order to prevent the destruction of the stomach wall, the levels of stomach acid must be closely regulated. In conjunction with gastrin, mucus is also secreted to line and protect the stomach from its high level of acidity.

Intrinsic factor is a vital glycoprotein produced in the stomach. In order for vitamin B-12 to be absorbed, intrinsic factor must be present.

Another sphincter, known as the pyloric sphincter, controls the release of chyme into the small intestine.

Small intestine (approximately twenty feet long and connects the stomach to the large intestine): It is named the small intestine due to its small diameter. Importantly, the vast majority of all digestion and absorption of nutrients occurs here.

The pyloric sphincter connects the stomach to the small intestines. Its major role is to control the release of chyme into the small intestine.

Duodenum: the first portion of the small intestine is wider than the remainder of the small intestine.
Jejunum: the middle section
Ileum: the final section that connects the small and large intestines

The small intestine is lined with mucosa, and it is folded over many times. Within the folds are finger-like projections called villi. Villi help trap food, and they are necessary for the processes of digestion and absorption. A representative image of the structure of the small intestine is shown in Figure 2.5. The folds, villi, as well as absorptive cells that line the villi expand the absorptive surface area of the small intestine. Importantly, the vast majority of all nutrients are absorbed here. Fat-soluble nutrients go into the lymph system, while the other nutrients are absorbed into the blood stream.

During the process of digestion, several organs are also associated with the small intestines:

The liver secretes bile, which is stored in the gallbladder. The gallbladder will store the bile until it receives a hormonal signal that fat has entered the small intestine. When this occurs, bile is released into the duodenum through the common bile duct. The bile begins to emulsify (break down) the fat into smaller pieces that can be mixed with water.

The pancreas releases a mixture of water, bicarbonate, and enzymes called pancreatic juice that breaks down carbohydrates, protein, and fats. The bicarbonate neutralizes the acidity of the chyme to protect the intestinal wall. Without the bicarbonate, the high level of acid would destroy the lining of the small intestine, resulting in the formation of an ulcer.

Glucagon and insulin are hormones also released by the pancreas to help with glucose regulation.

The ileocecal sphincter connects the small intestines to the large intestines. Much like the pyloric sphincter (see above), the role of the ileocecal sphincter is to prevent the contents of the large intestine from reentering the small intestine. As undigested food cannot be absorbed in the small intestine, it must pass through the ileocecal sphincter and into the large intestine (also called the colon).

Large intestine (the last section of the digestive tract): It is five feet long and known as the colon. It is divided into four sections: (1) ascending colon, (2) transverse colon, (3) descending colon, and (4) sigmoid. Relative to the small intestine, the large intestine does not play a major role in the digestion and absorption of nutrients. However, after leaving the small intestines, the remaining water and certain minerals (sodium, potassium, and chloride) can be absorbed in the large intestine. As the food waste passes through the large intestine, a semi-solid mass known as feces is produced. Feces remains in the rectum—the end portion of the large intestine—until muscular contractions push it into the anus to be expelled. The expulsion of feces from the body is controlled by two anal sphincters.

Within the colon, there are also many mucus-producing cells, the function of which is two-fold: First, the mucus holds the feces together. Second, the secreted mucus protects the intestine from bacteria. The bacteria in the colon are necessary for breaking down the remaining food products that entered from the small intestine as well as for breaking down of some forms of fiber.

Higher levels of good bacteria help to protect the body from disease-causing bacteria, and they are an important part of the immune system. Bifidobacterial and lactobacilli are two types of bacteria that are considered important to our health. While some foods naturally contain these bacteria, other food products have the bacteria added to them.

Probiotics are foods that contain these bacteria. Fermented milk and yogurt are examples of probiotics that can be marketed to improve your gut health and digestion.

Prebiotics refers to substances that stimulate bacterial growth in the large intestine.